1. Field of the Invention
The present invention relates in general to the field of telecommunications, more particularly to multi-carrier networks like OFDMA (Orthogonal Frequency Division Multiple Access) wireless networks, and even more particularly to the management of radio resources in such networks. Specifically, the present invention concerns a method and system for scheduling downlink data transmission into the available resources. Typical application scenarios for the present invention includes, but are not limited to, beyond-3G cellular systems.
2. Description of the Related Art
Cellular phone systems and portable/movable terminals based on cellular transmission have evolved in the past years from 1st generation systems, based on analogue narrowband Frequency-Division Multiple Access (FDMA) transmission, to systems based on digital narrowband Frequency and Time Division Multiple Access (FDMA/TDMA) transmission first (also referred to as the 2nd generation or 2G systems), and, later, to digital broadband Code-Division Multiple Access (CDMA) transmission systems (3rd generation or 3G systems).
Further evolution, especially for supporting higher data rates, can be based on the improvement of the spectral efficiency of the transmission system. However, given the limits on spectral efficiency, an increase in the transmission bandwidth is foreseen for future generations of cellular phones. When transmission bandwidth is increased, the receiver typically shows an increase in circuit complexity, which is dependent on the type of modulation and multiplexing. 3G systems based on the CDMA operate effectively on bandwidths up to several MHz. 10 MHz are often considered a practical upper limit for the bandwidth of low-cost commercial CDMA equipment using a Rake receiver or other Single Carrier (SC) receivers with a time-domain equalizer.
When the bandwidth of the transmission systems becomes larger than a few MHz, a multi-carrier modulation is often more suitable to keep implementation complexity low.
The OFDMA transmission technique is gaining popularity as a preferred technology in the Broadband Wireless Access (BWA) emerging standards, particularly in respect of portable/mobile communications. The demand for high data rates, combined with physical requirements such as supporting near and non line of sight operation, multipath propagation mitigation and operation with fading channels, requires a technology that can efficiently provide an appropriate answer.
The OFDM (Orthogonal Frequency Division Multiplexing) technique can efficiently handle multipath propagation and increase robustness against frequency selective fading or narrow-band interference. OFDMA combines the TDMA and FDMA schemes; the time domain is segmented into groups of OFDMA symbols, and each symbol is segmented into sub-carriers. The number of symbols and sub-carriers which are allocated to a single transmitter vary according to the transmitter's needs (i.e. according to the amount of traffic data to be transmitted). The transmission rate (i.e. the adopted modulation and coding scheme) on those carriers is set to meet the transmitter's needs and capabilities.
Examples of OFDMA transmission systems are those compliant to the IEEE 802.16 family of standards, which relate to fixed and mobile BWA.
Even if flexible bandwidth allocation and QoS (Quality of Service) mechanisms are provided in the IEEE 802.16 standard, the details of scheduling algorithms, admission control, and reservation management are left to the implementation.
U.S. 2005/0265223 discloses a method and apparatus for scheduling downlink channels in an orthogonal frequency division multiple access system. Terminals compute a plurality of channel capacities and search for a channel with a maximum capacity. The terminals send, to a base station, feedback information including a channel number and a capacity value of the channel with the maximum capacity. The base station performs a first channel allocation process for allocating a channel with an optimal capacity to each terminal on the basis of the feedback information. The base station performs a second channel allocation process for allocating an adjacent channel to a corresponding terminal using the window bit when the terminal is not allocated a channel in the first channel allocation process.
U.S. 2007/060178 discloses systems and methods of scheduling sub-carriers in an OFDMA system in which a scheduler takes into account channel conditions experienced by the communication devices to optimize channel conditions. The scheduler can partition a set of sub-carriers spanning an operating bandwidth into a plurality of segments. The segments can include a plurality of global segments each of which includes a distinct non-contiguous subset of the sub-carriers spanning substantially the entire operating bandwidth. One or more of the global segments can be further partitioned into a plurality of local segments that each has a bandwidth that is less than a channel coherence bandwidth. The scheduler determines channel characteristics experienced by each communication device via reporting or channel estimation, and allocates one or more segments to communication links for each device according to the channel characteristics.